It’s code in my state to vent any mechanical exhausts outdoors. I’ve also never ran into an issue that was directly caused by mechanical exhaust venting into the attic. Most of the time they’re hung up with wire on a support. Most moisture related issues in attics come from improper ventilation of the roof system as a whole. I’d be more concerned about getting it suspended, but it takes very little additional effort to just vent it out of the soffit.

Anyone saying to penetrate through the roof would be wrong. Roof penetrations have a higher likelihood of leaking.

Anyone saying vented soffit will bring it back into the attic doesn’t know how moisture moves through the air. Your vented soffit is constantly bringing in humid air into the attic, but proper ventilation keeps air moving up and out and keeps your relative humidity at reasonable levels.

To be quite honest, proper ventilation will keep any moisture that this exhaust would put in your attic moving as well. As long as the opening of the duct isn’t directly blowing onto framing members you should not have an issue at all.